I was happily using 19.07.10 on my FRITZ!Box 7530. Using PPPoE with a 300/300 line speed.
Speed tests show a solid 270 - 290Mbps upload and download.
I upgraded to 21.02.03 and noticed my download speeds went down to ~160Mbps and upload speeds down to around ~250Mbps.
I also noticed that the LAN ports on this release are backwards (e.g. LAN1 shows as LAN4).
Downgrading back to 19.07.10 caused the performance to go back to its original state.
Try to enable packet steering, it was shortly enabled by default in previous releases, but then disabled again, as its a mixed bag - helps on some systems, regresses on other (on quad core ipq40xx it has every opportunity to help). That aside, you're close to the hardware limits of ipq40xx, so even small differences will show up.
My ipq40xx device (AVM FB-4040) does easily handle 400 MBits download.
You could try
echo performance > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
as local startup script.
I will try this Thomas. I tried 19.07.10 on my 900/100 Fibre and I was easily getting 650Mbps down.
Clearly a problem with 21.02.